Fiber-optic current sensors are commonly based on the magneto-optic Faraday effect in an optical sensing fiber wound around the current conductor. Care has to be taken in the packaging of the sensing fiber so that mechanical stress does not obstruct the current measurement with the required accuracy via elasto-optic coupling. Typical accuracy requirements in electric power transmission systems are signal stabilities within ±0.2% or even ±0.1% over a temperature range, e.g. from −40° C. up to 85° C. There are essentially two types of sensing fiber used for fiber-optic current sensors;                (i) Low-birefringent sensing fiber ideally shows no intrinsic linear birefringence and therefore is fully sensitive to the magneto-optic Farady effect. However, bending the fiber to a sensing coil and hardening of the fiber coating at low temperatures introduce mechanical stress that can significantly influence the current measurement. Current sensors employing low-birefringent sensing fibers in some prior art accordingly comprise a stripped sensing fiber, i.e. an optical fiber without coating and residing is an oil-filled glass capillary.        (ii) In addition, prior art used spun highly birefringent sensing fiber [1]. Such a fiber is elliptically birefringent, which is achieved by local linear birefringence with principal axes that rotate along the fiber. Such fibers are typically produced by rotating the fiber preform during the drawing process. The local linear birefringence can be achieved by same fiber designs that are used for linear birefringent and thus polarization-maintaining fiber [2]. Such designs can be based on stress-induced birefringence such as panda, bow-tie [1], and elliptical cladding structures, geometrically induced birefringence such as in microstructured fibers [3], or combinations of stress and geometrically induced birefringence such as in elliptical core fibers [4]. The rotating intrinsic birefringence of a spun highly birefringent fiber basically makes the fiber less sensitive to external mechanical stress while the fiber still shows decent sensitivity to the magneto-optic Faraday effect. Accordingly, this fiber enables simplified fiber packaging compared to low-birefringent sensing fiber.        
However, the temperature dependence of the spun fiber's birefringence generally adds additional temperature dependent contributions to the overall signal. First, there are cacillatory signal instabilities as a function of temperature, resulting from interference of sets of light waves with temperature dependent phase difference [1]. Prior are uses several remedies to remove this oscillartory behaviour. Examples are the usage of a sufficiently long sensing fiber in connection with a broadband light source [1], then injection of pure eigenmodes into the sensing fiber [4], the use of a sensing fiber composed of two spun highly-birefringent fibers with opposite sense of spinning [10], and the use of a spun highly-birefringent fiber with small temperature dependent birefringence [3].
Secondly, the spun fiber's birefringence reduces the overall sensitivity of the current sensor compared to the corresponding sensor employing low-birefringent sensing fiber [1]. A change of birefringence of the spun fiber with temperature accordingly changes also the sensor's sensitivity and correspondingly the sensor's scale factor in many fiber-optic current sensor configurations [1]. This results typically in a mostly linear scale factor variation with temperature. Spun highly-birefringent fibers of prior art show a negative (or vanishing [3]) temperature dependence of their birefringence, i.e. their birefringence decreases with temperature and accordingly increases the scale factor of a current sensor employing this fiber. This linear or close to linear temperature dependent contribution is of the same sign and typically also of the same order of magnitude as temperature variation of the Faraday effect, which amounts to around 0.7%/100° C. for fused silica fiber [5]. Accordingly, without further means for temperature compensation, fiber-optic current sensors employing highly-birefringent spun fiber can have in total a temperature dependence that can reach up to a few percent/100° C.